I've made maybe a dozen of these sorts of pouches over the last fifteen years or so.
They really are very simple, and are a good way to start leatherwork.
You don't really need a pattern, just mark out a circle or oval for the bottom, then cut a band that will go all the way round, the width being the height of the finished pouch.
Sew it up inside out, turn it, punch holes round the neck of the bag for your thong.
I have used these for carrying money when I have a hole in my pocket (from a key or knife having nicked the cloth), since you can pass the thong under your belt and secure it with a small toggle. The bag sits inside the torn pocket.
An alternative, is to take a strip of thick leather about 1" (25mm) wide, and ten inches (250mm) long for the "base", and cut two ovals of more flexible leather to make a pouch that lies flatter to your chest.
Keith.